1. Field of the Invention
The present invention relates to optical systems and more particularly to a system in which coherent radiation having variations of intensity is smoothed to provide a more uniformly illuminated image plane. More particularly the invention contemplates a laser light source of coherent light in which the light beam is modified to provide a beam of more uniformity in cross section. Such beams of high intensity and relatively uniform illumination are especially useful in the manufactures of semiconductor wafers.
2. Description of the Prior Art
Increased resolution and throughput requirements in micromachining require use of powerful monochromatic light sources as provided by lasers. However, while monochromaticity of laser radiation helps to increase resolution, the high coherency reduces it as described in Principles of optics by Born and wolf, Oxford 1970 fourth edition. Thus, prior art systems have been designed to reduce or eliminate coherency.
U.S. Pat. No. 4,619,508 reduces the coherency by breaking the beam into many spatially shifted parts using either a staircase prism, fiberoptic bundle, or an array of glass rods of different thicknesses. This, however, introduces a local loss of light due to diffraction on the edges of the staircase prisms or glass rod arrays, or obscuration of light by bevels and rolloffs, which result in a high non-uniformity of illumination.
A similar problem exists in another system of destroying the coherency, described in U.S. Pat. No. 5,224,200. This system employs a coherency delay line installed between the laser and homogenizer. The delay line comprises two mirrors, one partially reflecting, and one totally reflecting, arranged so that light incident from the laser first strikes the partially reflecting mirror. A portion of the beam passes through and a portion is reflected back to the totally reflective mirror. The reflected portion is in turn reflected back to the partially reflecting mirror at a position spaced from the position of initial incidence, where again a portion is transmitted, and a portion is reflected. This process is repeated until the reflected beam traverses the partially reflective mirror and finally bypasses the partially reflective mirror altogether. The series of beams transmitted through the partially reflecting mirror and the final beam that bypasses it all together, are focused through a lens into the homogenizer. The partially reflective mirror coating is made in such a way that all portions of the light beam transmitted through it are substantially equal in intensity. This is achieved by making the thickness of partially reflective coating on the partially reflective mirror decreasing at each successive position of incidence by the beam.
The magnitude of the spatial shift between adjacent beams must be equal to or exceed their size, otherwise it can not enter or exit the system without the energy loss. Resulting energy distribution is very non-uniform because of voids between adjacent beams formed due to the nonuniformity of the input laser beam, edge diffraction or shear separation. To correct the nonuniformity of illumination a mirror funnel homogenizer is employed and a large number of reflections is required to correct the problem. Losses associated with each reflection, result in a low throughput of the device.